The production of glass-ceramic articles conventionally contemplates three general steps. First, a glass batch of a predetermined composition, and which commonly contains a nucleating agent, is melted. Second, the molten batch is simultaneously cooled and a glass body of a desired configuration is shaped therefrom. Third, the glass body is heat treated in a particular manner to cause the in situ growth of crystals. In the preferred heat treatment practice, the crystallization process is divided into two steps. Thus, the glass body will first be heated to a temperature in the vicinity of, or somewhat above, the transformation range of the glass to generate nuclei in the glass, after which the body will be raised to a higher temperature, customarily in excess of the softening point of the glass, to cause the growth of crystals on the nuclei.
Glass-ceramic articles are customarily highly crystalline, i.e., greater than 50% by volume crystalline, having a microstructure consisting of relatively uniformly-sized, fine-grained crystals homogeneously dispersed within a residual glassy matrix. The very high crystallinity of glass-ceramic articles gives rise to two intrinsic phenomena: (1) the chemical and physical properties thereof will be quite different from those demonstrated by the parent or precursor glass body, but will be closely similar to those of the crystal phase; and (2) the composition of the residual glassy matrix will be far different from that of the parent glass since the components incorporated within the crystal phase will have been taken therefrom. And, because glass-ceramic articles are produced through the crystallization in situ of glass article, they are non-porous and free of voids.
U.S. Pat. No. 2,920,971 provides an extensive explanation of the mechanics of production and a discussion of the microstructure of glass-ceramic articles. Reference is made to that patent for further information concerning the physical characteristics and internal structure of such articles, as well as the parameters involved in the nucleation and crystallization thereof.